(1) Field of the Invention
The present invention relates to a low-profile antenna structure, and in particular to an antenna structure that can electrically control the directivity thereof.
(2) Description of the Related Art
The directivity of an antenna can be changed by various methods, such as by spatially slanting and rotating the antenna, and using electricity. Examples of antennas known for employing the latter method are: a diversity antenna, which has multiple antennas with different directivities and chooses one of them; and an array antenna disclosed in Patent Reference 1 (Japanese Laid-Open Application No. 2002-118414).
Further, Patent Reference 2 (Japanese Laid-Open Application No. 2005-252406) discloses technology for making the directivity variable by magnetically coupling an excited element and a parasitic element provided on the back of a television receiver and the like.
The technology disclosed in Patent Reference 2 is effective when used in a situation where the direction from which the television receiver and the like receive an electromagnetic wave is limited to some extent. However, in the case of a mobile communication system, an antenna that has a strong directivity and does not limit the wave arrival direction is required since the Space Division Multiplexing technology (hereinafter, simply “SDM”) is applied to the system. Especially, the system requires technology that controls beam-forming and null-forming with great flexibility.
Moreover, in many cases, transceivers used in the mobile communication system are mobile devices, and hence are expected to become smaller. For example, antennas for RFID (Radio Frequency Identification) use have become smaller through the use of a high-frequency band at 2.45 GHz. Like in this case, an antenna element can be made smaller by using higher frequency bands. Thus, in prospect of the use of such higher frequency bands in the future, there will be a demand for an antenna structure that benefits from such a size advantage.
The antenna element of the antenna disclosed in Patent Reference 1 can be made smaller using high frequency bands. However, being composed of a dipole element or a monopole element, this antenna needs to be placed either (i) far enough from a metal case or a circuit board of the transceiver, or (ii) standing straight up on the case or the circuit board, which are regarded as ground planes. Either way, the antenna protrudes outwardly far from the transceiver, making the transceiver inconvenient to carry around.